An electroencephalogram (EEG) is a recording of the neuronal electrical activity of the cerebral cortex made by electrodes placed on the skull. EEG waves normally reflect the summed extracellular currents that result from the generation of synaptic potentials in the pyramidal cells and are thus a type of field potential. Because the currents generated by a single cell are too small to be detected as discrete events by an electrode on the skull (to record the activity of a single neuron, a microelectrode must be placed within microns of the neuron), the EEG waves reflect the combined activity of many pyramidal cells. Moreover, for the activity of a group of neurons to generate an event detectable on EEG, they must be oriented so that their individual currents summate to produce a detectable field. The arrangement of pyramidal neurons, with their apical dendrites aligned in parallel to form a dipole sheet, is particularly favorable for generating large field potentials. One pole of this sheet is oriented toward the cortical surface and the other toward the subcortical white matter. Thus the currents generated •Fig. 10.5 Electroencephalographictracingsduringdrowsiness;stages1,2,and4ofslow-wave(non–rapideyemovement[non-REM])sleep;andREMsleep.(ModifiedfromShepherdGM.Neurobiology. London:OxfordUniversityPress;1983.) by cortical pyramidal cells have a similar orientation and can therefore summate to produce a field potential that can be detected. The need for summation also explains why EEG signals reflect primarily synaptic potentials rather than action potentials: electrical events must overlap in time in order to sum, and synaptic potentials have much longer durations than do action potentials.
The sign of an EEG wave can be positive or negative, but its direction alone does not indicate whether pyramidal cells are being excited or inhibited. For instance, a negative EEG potential may be generated at the surface of the skull (or cortex) by excitation of apical dendrites or by inhibition near the somas. Conversely, a positive EEG wave can be produced by inhibition of apical dendrites or by excitation near the somas.
A normal EEG tracing consists of waves of various frequencies. The dominant frequencies depend on several factors, including the state of wakefulness, the age of the subject, the location of the recording electrodes, and the absence or presence of drugs or disease. When a normal awake adult is relaxed with the eyes closed, the dominant frequencies of the EEG recorded over the parietal and occipital lobes are about 8 to 12 Hz, the alpha rhythm. If the subject is asked to open the eyes, the wave becomes less synchronized, and the dominant frequency increases to 13 to 30 Hz, which is called the beta rhythm. The delta (0.5 to 2 Hz) and theta (3 to 7 Hz) rhythms are observed during sleep (see the following discussion;
Fig. 10.5 ). Also, brief EEG waves do exist and, because of their shape, are sometimes referred to as spikes, but this does not imply that they are associated with action potentials.
An EEG change that can be elicited by a stimulus is called a cortical evoked potential. A cortical evoked potential is best recorded from the part of the skull located over the cortical area being activated. For example, a visual stimulus results in an evoked potential that can be recorded best over the occipital bone, whereas a somatosensory evoked potential is recorded most effectively near the junction of the frontal and parietal bones. Evoked potentials reflect the activity in large numbers of cortical neurons. They may also reflect activity in subcortical structures.
Evoked potentials are small in comparison with the size of the EEG waves. However, their appearance can be enhanced by a process called signal averaging. In this process, the stimulation is repeated, and the EEGs recorded during each trial are electronically averaged. With each repetition of the stimulus, the evoked potential occurs at a fixed time after the stimulus. When the records are averaged, the components of the EEG that have a random temporal association with the stimulus cancel each other, whereas the evoked potentials sum.